Lu-Hf analyses of zircon from the Makoppa Dome and Amalia-Kraaipan area: 1 implications for evolution of the Kimberley and Pietersburg blocks of the Kaapvaal Craton. Marlina Elburg, Marc Poujol To cite this version: Marlina Elburg, Marc Poujol. Lu-Hf analyses of zircon from the Makoppa Dome and Amalia-Kraaipan area: 1 implications for evolution of the Kimberley and Pietersburg blocks of the Kaapvaal Craton.. South African Journal of Geology, Geological Society of South Africa, 2020, 123 (3), pp.369-380. 10.25131/sajg.123.0025. insu-02885429 HAL Id: insu-02885429 https://hal-insu.archives-ouvertes.fr/insu-02885429 Submitted on 30 Jun 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 1 Lu-Hf analyses of zircon from the Makoppa Dome and Amalia-Kraaipan area: 2 implications for evolution of the Kimberley and Pietersburg blocks of the 3 Kaapvaal Craton. 4 5 Marlina A. Elburg1, Marc Poujol2 6 1 Department of Geology, University of Johannesburg, South Africa; 7 [email protected] 8 2 Univ. Rennes, CNRS, Géosciences Rennes - UMR 6118, F-35000 Rennes, 9 France; [email protected] 1 10 Abstract 11 Previously dated zircon crystals from the Amalia-Kraaipan granite-greenstone belts 12 and Makoppa Dome were analysed for their Lu-Hf isotopic characteristics to refine 13 the geological evolution of these areas. Samples from the Makoppa Dome, belonging 14 to the Pietersburg Block, largely fall within the epsilon Hf-age range for granitoids 15 from the eastern part of the block. However, the oldest 3.01-3.03 Ga trondhjemitic 16 gneisses show that reworking of juvenile mafic crust started earlier in the western 17 than the eastern part of the block, suggesting a diachronous tectonic evolution. The 18 three granitoids from the Amalia-Kraaipan area fall within the field for Pietersburg and 19 Kimberley block granitoids. Contribution from older crustal material is seen in a 3.08 20 Ga schist, likely derived from a volcanic protolith, from the Madibe Belt, in the far east 21 of the Kimberley Block, with a mantle extraction age of 3.25-3.45 Ga. The data 22 suggest that the Kimberley Block, like the Pietersburg Block, also contains (minor) 23 ancient crustal components, derived from a depleted mantle source prior to 3.1 Ga. 24 The new data suggest that the Kimberley and Pietersburg blocks underwent a very 25 similar Paleo- to Mesoarchean crustal evolution, with a major crust formation event at 26 3.1-3.0 Ga followed by successive crust reworking until 2.77 Ga. Lavas of the 27 Ventersdorp Supergroup, for which zircons from a ca. 2.75 lapilli tuff give Hfi of +2, 28 are the first evidence of a juvenile source, after 300 Myr of crustal reworking. 29 30 Introduction 31 The Kaapvaal Craton is one of the better-studied Archean cratons, thanks to its 32 relatively accessible and well-exposed geological record. Its division into four 33 separate terranes or blocks (Eglington and Armstrong, 2004), based on 34 geochronological data, is now widely accepted (Figure 1), with the Swaziland Block 35 having attracted the most academic attention as it hosts the craton’s oldest rocks at 36 3.66-3.70 Ga (Compston and Kröner, 1988; Kröner et al., 1996; Robb et al., 2006; 37 Zeh et al., 2011). Recorded ages are somewhat younger in the Witwatersrand and 2 38 Pietersburg blocks (Anhaeusser, 2019) at ≤3.34 Ga (Laurent and Zeh, 2015; Poujol 39 and Anhaeusser, 2001). Because of its extensive cover, the Kimberley Block is the 40 least known, and virtually no ages older than ca. 3.2 Ga have been found 41 (Anhaeusser and Walraven, 1999; Cornell et al., 2018; Poujol et al., 2008). The 42 boundaries between the four terranes are based on geophysically defined 43 lineaments, which may or may not have any surface expression, as well as 44 recognisable faults and shear zones, and extensions thereof (Eglington and 45 Armstrong, 2004). As such, they are somewhat dependent on changing 46 interpretations of the geophysical data (Corner and Durrheim, 2018), additional 47 geochronological information and also geochemical data. In the latter respect, the 48 development of the past fifteen years of laser ablation multi-collector inductively 49 coupled mass spectrometry (LA-MC-ICPMS), which permits the determination of the 50 Hf isotopic characteristics of the same zircon grains that provide the age of 51 intrusions, needs to be mentioned. Zircon is the most robust carrier of U-Pb age data, 52 and has therefore been the mineral of choice to obtain reliable geochronological 53 information. Zircon’s high contents of the element hafnium, a geochemical twin of 54 zirconium, and limited amount of the radioactive parent element lutetium, makes 55 them also the best carrier of Hf isotopic information. As mantle and crust have 56 contrasting Lu/Hf ratios, they develop different Hf isotope ratios over time. Studying 57 the Hf isotopic compositions of dated zircon crystals therefore allows us to test 58 whether intrusions with the same age also have similar magma sources; and whether 59 these sources were juvenile (mantle-derived), or incorporated older crustal materials. 60 As isotopic information, unlike whole rock geochemistry, is only affected by the 61 sources contributing to the magma, irrespective of crystal fractionation, it is a robust 62 test for similarity among rocks of the same age. This technique has been applied 63 successfully to a number of plutons of the Kaapvaal Craton (see overviews in Kröner 64 et al., 2019 and Laurent et al., 2019, and the many works by Zeh and coworkers (e.g. 65 2009, 2011)), but a great number of intrusives of which the age is known has not 3 66 been analysed for the Hf isotopic composition. In addition to providing information on 67 the petrogenesis of the igneous rocks from which the zircon grains are derived, 68 establishing an age-Hf isotopic zircon database for the Kaapvaal Craton is also 69 important for the study of detrital zircon in sedimentary rocks, which provides 70 information on the sediments’ provenance. 71 To add to the available information on the geological evolution of the Kimberley and 72 Pietersburg blocks and Kaapvaal Craton zircon age-Hf isotopic database, we 73 performed Lu-Hf isotopic analyses on zircon samples of ten igneous rocks from the 74 Amalia-Kraaipan area of the Kimberley Block and the Makoppa Dome of the 75 Pietersburg Block that were analysed for their U-Pb characteristics previously 76 (Anhaeusser and Poujol, 2004; Poujol et al., 2002; Poujol et al., 2008; Poujol et al., 77 2005). Our new data are compatible with the interpretation that the Makoppa Dome is 78 indeed part of the Pietersburg Block, and provides the earliest evidence for reworking 79 of juvenile mafic material in the region, whereas they add to our relatively poor 80 knowledge of the Kimberley Craton. Additionally, these data are compared to the 81 existing detrital zircon database for Archean-Paleoproterozoic Kaapvaal Craton and 82 provide matches for zircon grains from the Witwatersrand Supergroup and Waterberg 83 Group. 84 85 Geological background 86 The geology of the Pietersburg Block has mainly been studied in its eastern part, 87 between the extension of the Palala Shear Zone and the Thabazimbi-Murchison 88 Lineament (Figure 1c), which are interpreted to be the northern and southern 89 boundary, respectively, of the Pietersburg Block (Eglington and Armstrong, 2004). 90 The Southern Marginal Zone of the Limpopo Belt, which is located north of the Hout 91 River Shear Zone (Figure 1c), is also deemed part of the Pietersburg Block 92 (Eglington and Armstrong, 2004). The western part of the block, in which the 93 Makoppa Dome is located, is separated from the eastern part by the ca. 2.05 Ga 4 94 Bushveld Complex, which straddles the boundary of the Witwatersrand and 95 Pietersburg blocks. In its eastern part, the Pietersburg Block consists of ENE-WSW 96 running greenstone belts (the Murchison, Giyani and Pietersburg belt), with several 97 generations of granitoids. The oldest of these are the Goudplaats-Hout River tonalite- 98 trondhjemite-granodiorite (TTG) gneisses at 3.2-3.43 Ga (Laurent and Zeh, 2015). 99 Further TTG intrusions followed from 3.0 to ca. 2.8 Ga, and more K-rich magmatism 100 from 2.9 to 2.67 Ga (see Laurent et al., 2019 for a review). Magmatism has been 101 interpreted to reflect accretion of several arcs, and a >3.2 Ga continental nucleus, to 102 the proto-Kaapvaal Craton, consisting of the Swaziland and Witwatersrand blocks. 103 The Makoppa Dome straddles the boundary between South Africa and Botswana, 104 with an area of ca. 7600 km2. The only published work on the area is the paper by 105 Anhaeusser and Poujol (2004), which also provides the ages of the zircon used for 106 the present study. The rocks of the Makoppa Dome are largely obscured by post- 107 Mesozoic sediments, but consist of both granitoids and greenstones (Figure 1b). The 108 latter are amphibolites, serpentinite, talc/chlorite schists and banded iron formations. 109 The granitoids, which were the focus of the U-Pb zircon and whole rock geochemistry 110 study by Anhaeusser and Poujol (2004), are the Vaalpenskraal trondhjemite/tonalite 111 gneiss, the Makoppa granodiorite/monzogranite and the Rooibokvlei 112 granodiorite/monzogranite.